A good look at a near-Earth visitor

The asteroid 1996 HW1 is a chunk of rock over 3.5 kilometers across. Its 3-year orbit around the Sun is a little odd: it’s elliptical, going out as far as the main asteroid belt, but then dipping back in to get only 19 million kilometers or so from the Earth’s orbit. This makes it a Near Earth Object, or NEO, though not really a dangerous one. It belongs to a class of asteroids called Amors, which have similar orbits.

The orbit of 1996 HW1 is in blue. In September 2008 it dove near the Earth in its elliptical orbit, passing about 20 million km away. Click to get lots more info.

But what does it look like? What shape is it? It’s too small to be resolved even by big telescopes, so you might think we can’t determine its shape.

But how did they get the shape of this thing? Between July 2008 and January 2009, Michael Hicks, Heath Rhoades, and James Somers observed the asteroid over many occasions. As 1996 HW1 moves around the Sun, the amount of sunlit surface we see changes (just like the Moon goes through phases). If it were a perfect sphere, then we can predict exactly how much light we would see from it as our angle to it changes. But then, we could do that if it were, say, a cube, too! Or a banana-shape, or an axisymmetric epicycloid (a figure-8 shape that has loomed large in my life; remind me to tell you that story someday).

In fact, it’s possible, given enough observations, to determine the rough shape of any object from measuring how its brightness changes over time. This method is called light curve inversion. A light curve is the plot of brightness over time. You feed those numbers into some pretty fierce equations which determine the shape of the object. Since normally you use the shape to get the light curve, this method is the inverse of that. That’s where the name comes from.

In the animation you can see that HW1 is elongated. I also noticed that the rotation period (mentioned in the Table Mountain page) is long for a small asteroid, about 9 hours. Most small asteroids spin faster than that, which is interesting. Why so slow? I suspect it’s suffering from the YORP effect, where sunlight can slow the spin of an asteroid. This is strongest in asymmetric rocks, and HW1 is clearly not terribly symmetric, so this may indeed be the case.

This method of shape fitting is incredibly powerful. We can learn a lot from just a few observations, and of course the more data we get, and the more accurate they are, the better a fit we can get to an asteroid’s shape, and the more we can learn about them. And need I remind you, these rocks are not all safe. Some of them have orbits which do cross ours, and one the size of HW1 is big enough to cause a mass extinction. So hear me well when I say that the more we know about these asteroids, the better.

And all of this can be learned without ever leaving the comfy confines of our little blue planet! So when we do venture out to visit these interplanetary rogues, we’ll have a much better idea of what they’re about.

And that is the shape of things to come.

Tip o’ the Whipple Shield to Heath Rhoades for letting me know about his work!